1. Field of the Invention
The present invention relates to exhaust gas recirculation (EGR) systems for internal combustion engines. More specifically, the invention is directed to EGR systems of the type which recirculate at least a portion of the engine exhaust gases into the engine air intake system for the purpose of reducing NOx emissions.
2. Description of Related Art
With continued tightening of governmental regulations on vehicular exhaust emission, particularly NOx, not only has the need to recirculate exhaust gases back to the engine intake become apparent, but so has the need to improve upon existing EGR technology.
U.S. Pat. No. 4,217,869 to Masaki discloses an EGR system in which combustion gases are forced from a reaction chamber through an outlet port into an intake passageway by either an ejector effect or suction produced by the engine exhaust gases drawn from an outlet portion of an EGR passageway.
Likewise, commonly owned, co-pending U.S. patent application Ser. No. 08/152,453 discloses an exhaust gas recirculation system in which a venturi or ejector tube is used to create a pressure differential across the EGR tube to drive the exhaust gases into the engine intake passageway.
However, such systems, when used on engines having efficient turbomachinery and/or an EGR cooler, especially on heavy duty engines, face the problem that an exhaust-to-intake pressure differential can occur that is either too low or unfavorable. This is particularly the case at rated speed and high loads where EGR rates near 20% may be required, necessitating EGR flow rates beyond that which simple venturi or ejector aided induction systems can supply.
The deficiencies of pressure differential type EGR induction systems have been recognized for some time. In U.S. Pat. No. 4,196,706 to Kohama et al., control valves are used to regulate the quantity of exhaust gas that is recirculated, and in recognition of the fact that insufficient ERG pressure may exist under certain operating conditions, Hamai U.S. Pat. No. 4,276,865 teaches the use of an engine-driven pump upstream of the EGR control valve for insuring that sufficient pressure exists to introduce the EGR gases into the engine intake passageway. However, the use of an engine-driven pump adds to the cost and weight of the EGR system, and is a source of parasitic losses.
Thus, the need still exists for a simple and inexpensive means for insuring that sufficient pressure exists to introduce the EGR gases into the engine intake passageway under all conditions, and particularly on turbocharged diesel engines.
As described in an article entitled "Parameter Effects on Mixer-Ejector Pumping Performance" (Skebe et al., AIAA-88-0188, American Institute of Aeronautics and Astronautics, 1988) ejectors have been used to improve aircraft performance in a variety of ways, including engine component cooling, thrust augmentation, and exhaust noise and temperature reduction. In this context, and particularly for advanced turbofan applications, a substantial increase in pumping performance of an ejector system has been found to be obtainable through the use of low loss "forced" mixer lobes. However, such lobed mixer type ejectors have not been used in land vehicle applications, especially with land vehicle engines, such as diesel engines, and particularly not in connection with EGR systems for such engines, either with or without exhaust driven turbocompressors.